1. Field of the Invention
The present invention relates to a manufacturing method of a honeycomb structure for use in a diesel particulate filter (DPF) and the like, in which one end portion of each cell is plugged.
2. Description of the Related Art
An exhaust gas discharged from an internal combustion engine such as a diesel engine includes a large amount of particulates (particulate substances) containing, as a main component, carbon which is a cause for environmental pollution, and hence a filter for trapping (filtering) the particulates is generally mounted on an exhaust system of the engine.
Usually, in the filter for use in such a purpose, there is used a plugged honeycomb structure 1 including, as shown in FIGS. 2(a) and (b), a honeycomb structure 2 in which a plurality of cells 9 are separated by porous partition walls 7 and formed so as to communicate with two end faces, and plugging portions 11 arranged in complementary checkered patterns on one end face side and the other end face side of the honeycomb structure 2 so as to plug one of two opening ends of each of the cells 9.
After the exhaust gas flows into the filter including such a plugged honeycomb structure from one end face 3 of the filter and the particulates and the like included in the gas are removed from the gas, the gas flows out of the other end face 5. Specifically, the exhaust gas first flows into cells 9b having end portions which are not closed at the one end face 3 of this filter and having closed end portions at the other end face 5, passes through the porous partition walls 7, moves to cells 9a having closed end portions at the one end face 3 and end portions which are not closed at the other end face 5, and is discharged from these cells 9a. Then, in this case, the partition walls 7 function as filter layers, and the particulates in the gas are trapped by the partition walls 7 and deposited on the partition walls 7.
As a manufacturing method of the above plugged honeycomb structure, a method is known in which a transparent sheet or the like is attached to an end face of a formed honeycomb body, an image of the end face is picked up, then the picked-up image is subjected to image processing to recognize positions of the cells, positions of the sheet corresponding to the cells to be plugged are subjected a perforation treatment by laser processing or the like based on the recognized positions, then an end portion of the honeycomb structure is submerged into a plugging slurry including a slurried material of plugging portions, and end portions of the cells to be plugged are filled with the plugging slurry from holes made by the perforation treatment in the sheets (e.g., see Patent Document 1).
In this manufacturing method, as described above, the positions of the cells are recognized by the image recognition. Specifically, the image of the end face of the honeycomb structure picked up by a charge-coupled device (CCD) camera or the like is binarized. When a lump of black images in the binarized image have a predetermined cell shape (e.g., a quadrangular shape), or reaches a certain ratio (e.g., 80% or more) of a predetermined cell area, the images are regarded as the cells, and the positions of the cells are recognized. It is to be noted that images which do not have the predetermined cell shape and which do not reach the certain ratio of the predetermined cell area are regarded as foreign matters such as dust attached to the end face of the honeycomb structure, and the positions are not subjected to the perforation treatment by mistake.
However, among the cells, the cells positioned at an outermost peripheral portion of the honeycomb structure (cells which come in contact with an outer peripheral wall of the honeycomb structure) have a partially chipped shape as compared with the other cells, and areas of the cells are reduced. Therefore, in the above image processing, positions of the cells are not recognized as the cells in many cases. Even when the cells should originally be plugged, the positions of the sheet corresponding to the cells are not subjected to the perforation treatment, and the cells are not filled with the plugging slurry. As a result, any plugging portion is not formed, and cells having both end portions which remain to be opened are left.
In such cells having both the end portions which remain to be opened, a large part of the particulates in the exhaust gas which has flowed into the cells is not trapped, and passes through the honeycomb structure. Therefore, the presence of such cells is a factor for deterioration of a performance of the filter.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-300922